Implantable cardiac pacemakers are commonly placed in a subcutaneous pocket and coupled to one or more transvenous medical electrical leads carrying pacing and sensing electrodes positioned in the heart. A cardiac pacemaker implanted subcutaneously may be a single chamber pacemaker coupled to one medical lead for positioning electrodes in one heart chamber, atrial or ventricular, or a dual chamber pacemaker coupled to two leads for positioning electrodes in both an atrial and a ventricular chamber. Multi-chamber pacemakers are also available that may be coupled to three leads for positioning electrodes for pacing and sensing in one atrial chamber and both the right and left ventricles.
The pacing mode of a pacemaker is commonly designated by a three- or four-letter code. The first letter may be an “A” for atrial, “V” for ventricular or “D” for dual (atrial and ventricular) to indicate which chamber(s) are paced. The second letter indicates which chamber(s) are sensed again designated by an “A,” “V,” or “D.” The third letter indicates how the pacemaker responds when it senses a cardiac event. For example, a sensed cardiac event may trigger a pacing pulse and is indicated by a letter “T” or a sensed cardiac event may inhibit a pacing pulse and is indicated by a letter “I.” In some cases, sensed cardiac events may both trigger and inhibit a pacing pulse depending on which chamber the event was sensed in. This dual response is indicated by the third letter “D.”
By this convention, an AAI pacing mode delivers pacing pulses in an atrial chamber, senses atrial P-waves attendant to atrial depolarization in the atrium, and inhibits the atrial pacing pulse when a P-wave is sensed. A VVI pacing mode delivers pacing pulses in a ventricular chamber, senses ventricular R-waves attendant to ventricular depolarization, and inhibits the ventricular pacing pulse when the R-wave is sensed.
One pacing mode is a DDD mode, which includes dual chamber pacing in both an atrial and ventricular chamber, dual chamber sensing in both an atrial and ventricular chamber, and a dual response to sensed events. For example, a sensed P-wave may trigger a ventricular pacing pulse at a programmed atrioventricular (AV) interval (atrial-triggered ventricular pacing), but a sensed R-wave during the AV interval inhibits the ventricular pacing pulse. Available dual chamber pacemakers that are capable of performing DDD pacing are implanted in a subcutaneous pocket and coupled to a transvenous atrial lead and a transvenous ventricular lead carrying atrial pacing and sensing electrodes and ventricular pacing and sensing electrodes, respectively, to enable the dual chamber pacemaker to sense and pace in both chambers.
A fourth letter, R, may be used to designate a rate responsive pacing mode of the pacemaker. In rate responsive pacing, the pacing rate is automatically adjusted in response to a sensor that indicates the metabolic need of the patient, such as a patient activity sensor. An AAIR, VVIR, or DDDR pacing mode is configured to provide rate responsive pacing in the indicated heart chamber(s) by automatically adjusting the pacing lower rate according to a sensor signal indicating the patient's metabolic need.